Numerous proposals have already been made to place a lens facing an optical fiber for the purpose of providing coupling between the fiber and a phototransducer.
By way of example, the following documents teach various different ways of using such coupling lenses: DE-A-3632743; FR-A-2548392; IEEE Journal of Quantum Electronics, Vol. 16, No. 10, October 1980, New York, U.S., pages 1067-1072; Yamada et al., "Characteristics of a hemispherical microlens for coupling between a semiconductor laser and single-mode fiber" pages 1067-1068; Patent Abstracts of Japan, Vol. 9, No. 262; and Applied Optics, Vol. 29, No. 18, Jun. 20, 1990, New York, U.S., pages 2692-2695.
The present inventors have already proposed improvements to those known devices in French patent application No. 91 11493 filed Sep. 18, 1991 and entitled (in translation) "Improved optical fiber for coupling with a phototransducer, and method of preparation".
Thus, accompanying FIG. 1 shows a device in accordance with the teaching of prior patent application No. FR-91 11493.
In accompanying FIG. 1, there can be seen an optical fiber 10 comprising a core 12 surrounded by cladding 14, together with a phototransducer 20 in alignment with the core 12 and a microlens 30 welded onto the cleaved end face 16 of the optical fiber 10 over its core 12.
A detailed study has shown that the parameters which influence the optical power coupled in a fiber of given type and provided with a collecting lens 30 are numerous, and comprise, in particular:
the wavelength of the radiation;
the angular aperture of the beam;
the radius of curvature of the collecting lens 30;
the accuracy with which it is centered on the core of the fiber 10;
the distance dw between the phototransducer 20 and the lens 30; and
the transverse positioning offset of the phototransducer 20 facing the core 12 of the fiber (assembly error).